The invention relates to the testing of semiconductor devices, particularly at sub 0.degree. C. temperatures, in wafer form. Typically, semiconductor devices are tested over a broad temperature range. Such environmental testing is commonly done on assembled devices, but this is undesirable because the wafers must be processed into finished assembled components before such testing can be accomplished. Such delays in obtaining data on material are objectionable. Testing assembled devices is also slo becuse it takes time to stabilize the packaged IC chip temperature to obtain temperature extreme data. It is much more desirable to temperature cycle an entire wafer and to probe the circuits thereon in the usual manner. However, a number of troublesome problems must be overcome. At the lower temperature moisture can collect on the wafer and on the viewing optics. This moisture is undesirable in that it can interfere with electrical testing and obscure viewing of the wafer. It is further desirable to electrically shield the test station to avoid electro magnetic interferance with the test operation.
One solution that has been used is to blow dry nitrogen gas over the wafer in the test fixture. This prevents moisture condensation, but makes it difficult to regulate wafer temperture. Alternatively, it has been proposed to evacuate the test station environment, but this requires an expensive housing and the application of a vacuum cycle slows down the system operation.